CN Bond Cleavage and Ring Expansion of N‐Heterocyclic Carbenes using Hydrosilanes

Schmidt, David; Berthel, Johannes H. J.; Pietsch, Sabrina; Radius, Udo

doi:10.1002/anie.201204333

Cited by 132 publications

(96 citation statements)

References 103 publications

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“…4), where ring insertion requires migration of at least one methyl group to the carbenic carbon, it is found that the favoured pathway involves initial hydride migration (T1) followed by ring expansion (T2) and a final methyl migration (T3). 5,6 Trends in the T1 barriers (H migration) for HBMe 2 are similar to that of Ph 2 SiH 2 and H 2 BMe, being lowest for 2 (74 kJ mol −1 ) and 5 (105 kJ mol −1 ), with the other three NHCs having higher barriers of 132 to 150 kJ mol −1 . The preference for hydride migration has been consistently predicted by theoretical studies, [9][10][11][12][13] and also observed experimentally.…”

Section: Boranesmentioning

confidence: 66%

“…1 Despite NHCs being considered to be robust spectator ligands in catalysis, the activation of C-N bonds has significant generality, being thus far observed for the elements Be, 2 B, [3][4][5] Si, 6 Ni, 7 and Zn. 1 Despite NHCs being considered to be robust spectator ligands in catalysis, the activation of C-N bonds has significant generality, being thus far observed for the elements Be, 2 B, [3][4][5] Si, 6 Ni, 7 and Zn.…”

Section: Introductionmentioning

confidence: 99%

“…5,6 Thus far, theoretical studies with NHCs have almost exclusively focused on unsaturated NHCs with the standard structure 1 (Fig. 5,6 Thus far, theoretical studies with NHCs have almost exclusively focused on unsaturated NHCs with the standard structure 1 (Fig.…”

Section: Introductionmentioning

confidence: 99%

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Effects of the electronic structure of five-membered N-heterocyclic carbenes on insertion of silanes and boranes into the NHC C–N bond

2015

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The effect of varying the N-heterocyclic carbene (NHC) ligand on ring expansion and endocyclic C-N activation of NHCs by silanes and boranes has been investigated with theoretical methods. Five common NHCs were considered, including unsaturated, saturated, 3,4-methyl, 3,4-chloro substituted five-membered NHCs and a fused benzimidazole NHC. The substrates considered were Ph2SiH2 (with two hydrides migrating to the carbene carbon), H2BMe (two hydrides migrating), HBMe2 (hydride migration, followed by methyl migration) and BMe3 (two methyls migrating). Consistent with experimental observations, it was found that the kinetic barrier is lowest for the saturated NHC. The barrier is also quite low for the benzimidazole NHC. It was determined that the partial positive charge on the central carbene carbon in the initial adduct is inversely proportional to the magnitude of the activation barrier.

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Section: Boranesmentioning

confidence: 66%

Section: Introductionmentioning

confidence: 99%

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Effects of the electronic structure of five-membered N-heterocyclic carbenes on insertion of silanes and boranes into the NHC C–N bond

2015

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“…Thus, we strongly believe that the emergence of the cross‐product originates from H/D exchange between the starting material ( i Pr 2 Im) ⋅ AlH 3 ( 3 ) and ( i Pr 2 Im) ⋅ AlD 3 ( [D 3 ]‐3 ). Furthermore, we assume a reaction mechanism as shown in Scheme : In the first step the bis‐NHC adduct is formed, followed by NHC insertion into one of the Al−H bonds to give intermediate A . Subsequent amido transfer from the NHC carbon atom to the aluminum atom leads to intermediate B , which finally reacts by a hydride shift from the aluminum atom to the (former) carbene carbon atom with the formation of the product 6 .…”

Section: Resultsmentioning

confidence: 99%

Reactivity of NHC Alane Adducts towards N‐Heterocyclic Carbenes and Cyclic (Alkyl)(amino)carbenes: Ring Expansion, Ring Opening, and Al−H Bond Activation

Schneider

Hock

Bertermann

et al. 2017

Chemistry A European J

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The synthesis of mono‐NHC alane adducts of the type (NHC)⋅AlH3 (NHC=Me2Im (1), Me2ImMe (2), iPr2Im (3 and [D3]‐3), iPr2ImMe (4), Dipp2Im (10); Im=imidazolin‐2‐ylidene, Dipp=2,6‐diisopropylphenyl) and (NHC)⋅AliBu2H (NHC=iPr2Im (11), Dipp2Im (12)) as well as their reactivity towards different types of carbenes is presented. Although the mono‐NHC adducts remained stable at elevated temperatures, ring expansion occurred when (iPr2Im)⋅AlH3 (3) was treated with a second equivalent of the carbene iPr2Im to give (iPr2Im)⋅AlH(RER‐iPr2ImH2) (6). In 6, {(iPr2Im}AlH} is inserted into the NHC ring. In contrast, ring opening was observed with the sterically more demanding Dipp2Im with the formation of (iPr2Im)⋅AlH2(ROR‐Dipp2ImH2)H2Al⋅(iPr2Im) (9). In 9, two {(iPr2Im)⋅AlH2} moieties stabilize the ring‐opened Dipp2Im. If two hydridic sites are blocked, the adducts are stable with respect to further ring expansion or ring opening, as exemplified by the adducts (iPr2Im)⋅AliBu2H (11) and (Dipp2Im)⋅AliBu2H (12). The adducts (NHC)⋅AlH3 and (iPr2Im)⋅AliBu2H reacted with cAACMe by insertion of the carbene carbon atom into the Al−H bond to give (NHC)⋅AlH2/iBu2(cAACMeH) (13–18) instead of ligand substitution, ring‐expansion, or ring‐opened products.

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“…Next, we reduced 5 with KC 8 (5 equiv). [20,21] To the best of our knowledge,r ing expansion through the regioselective insertion of borylene into the CÀNb ond of the C 3 N 2 fivemembered ring has never been described until now. The 11 BNMR spectrum of 4 displayed as inglet at 9.60 ppm.…”

mentioning

confidence: 94%

Ring Expansion, Photoisomerization, and Retrocyclization of 1,4,2‐Diazaboroles

Ganguly

et al. 2017

Angew Chem Int Ed

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CN Bond Cleavage and Ring Expansion of N‐Heterocyclic Carbenes using Hydrosilanes

Cited by 132 publications

References 103 publications

Effects of the electronic structure of five-membered N-heterocyclic carbenes on insertion of silanes and boranes into the NHC C–N bond

Effects of the electronic structure of five-membered N-heterocyclic carbenes on insertion of silanes and boranes into the NHC C–N bond

Reactivity of NHC Alane Adducts towards N‐Heterocyclic Carbenes and Cyclic (Alkyl)(amino)carbenes: Ring Expansion, Ring Opening, and Al−H Bond Activation

Ring Expansion, Photoisomerization, and Retrocyclization of 1,4,2‐Diazaboroles

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